This is a simple queue structure, but it demostrates 2 things, private data in a class, and having both a construtctor and a factory function that do the exact same thing

queue.js

function Queue(){
  return createQueue();
}
function createQueue() {
  const queue = [];
  const q = Object.create(Queue.prototype, { 
   constructor: {
		configurable: true,
        enumerable: false,
        value: Queue,
        writable: true,
   }
  });
  const methods = {
    enqueue(item){ return queue.push(item); },
    dequeue() { return queue.shift() },
    peek() { return queue[0] },
    debug() { return queue }
  }
  Object.assign(q, methods);
  Object.defineProperty(q, 'length', { get(){ return queue.length } })
  return Object.create(q);
}